Repeaters are set up in areas where signals transmitted from main transmitters are received weak to resolve instable and weak signal reception.
FIG. 1 shows a typical repeating system with repeaters using different frequencies.
Referring to FIG. 1, a main transmitter 101 transmits signals of frequency A, and repeaters 102 to 105 retransmit signals of frequencies B, C, D and E, respectively, which are different from the frequency A. In the conventional repeating system, different frequencies B, C, D and E are used for the repeaters 102 to 105, respectively. Since a plurality of frequency bands are used, the system requires many frequency resources and it is quite inefficient in the respective of frequency utility.
FIG. 2 shows another conventional repeating system which using the same frequency among repeaters.
A main transmitter 201 transmits signals of frequency A, and on-channel repeaters 202 to 205 retransmits signals through the frequency A, too. For on-channel repeaters to use the same frequency as a main transmitter, signals of the same frequency transmitted from the main transmitter 201 and the on-channel repeaters 202 to 205 should be able to be identified, individually.
If signals of the same frequency band are not the same (that is, the output signals of a repeater are different from the output signals of the main transmitter), the signals are on-channel interference signals at each repeater and they cannot be removed even with an equalizer or other devices.
Also, signals transmitted from the main transmitter and the on-channel repeaters have a time delay longer than a predetermined reference value, an equalizer cannot remove the delayed signals. Therefore, for on-channel repeating, the output signals of an on-channel repeater should be the same as the output signals of the main transmitter and the time delay between the output signals of the two parts should be short.
Shortcomings of the conventional repeaters will be described hereafter with reference to FIGS. 3 to 7.
FIG. 3 shows a typical RF amplifying on-channel repeater. Referring to FIG. 3, a reception antenna 301 and an RF receiver 302 receive RF signals transmitted from a main transmitter, and an RF band pass filter 303 passes through only signals of a predetermined band among the received RF signals. A high-power amplifier 304 amplifies the RF signals that have passed through the band, and the amplified RF signals are transmitted into on-channel through a transmission antenna 305.
FIG. 4 illustrates a conventional IF converting on-channel repeater. Referring to FIG. 4, a reception antenna 401 and an RF receiver 402 receive RF signals transmitted from a main transmitter, and an intermediate frequency (IF) down-converter 403 converts the received RF signals into IF signals based on a reference frequency supplied from a local oscillator (LO) 408. An IF band pass filter 404 passes through signals of a predetermined band among the IF signals, and an RF up-converter 405 converts the IF signals of the predetermined band that have passed through the band into RF signals based on a reference frequency supplied from the local oscillator 408. A high-power amplifier 406 amplifies the RF signals, which are transmitted out through a transmission antenna 407.
FIG. 5 illustrates a conventional on-channel repeater employing a Surface Acoustic Wave (SAW) filter. Referring to FIG. 5, a reception antenna 501 and an RF receiver 502 receive RF signals transmitted from a main transmitter, and an IF down-converter 503 converts the received RF signals into IF signals based on a reference frequency supplied from a local oscillator 508. A SAW filter 504 passes through signals of a predetermined band among the IF signals, and an RF up-converter 505 converts the IF signals of the predetermined band that have passed through the band into RF signals based on a reference frequency supplied from the local oscillator 508. A high-power amplifier 506 amplifies the RF signals, which are transmitted through a transmission antenna 507.
The on-channel repeaters of FIGS. 3 to 5 have a property that the characteristics of their output signals are inferior to those of input signals, because noise and multipath signals caused on a transmission path between the main transmitter and the on-channel repeaters, feedback signals generated due to low isolation of the transmission and reception antennas, and system noise occurring in the on-channel repeater system cannot be eliminated. Also, there is another problem in that the feedback signals generated due to the low isolation of the transmission and reception antennas restrict the transmission output power of the on-channel repeaters.
FIG. 6 shows a conventional on-channel repeater executing modulation and demodulation. Referring to FIG. 6, a reception antenna 601 and an RF receiver 602 receive RF signals transmitted from a main transmitter. An IF down-converter 603 converts the received RF signals into IF signals based on a reference frequency supplied from a local oscillator 611. A demodulator 604 demodulates the IF signals into baseband signals, and an equalizing and Forward Error Correction (FEC) decoding unit 605 eliminates noise and multipath signals caused on a transmission path between a main transmitter and the on-channel repeater and feedback signals generated due to low isolation of transmission and reception antennas from the baseband signals obtained from the demodulation. An FEC encoder 606 performs error correction coding onto the output signals of the equalizing and FEC decoding unit 605. A modulator 607 converts the FEC-encoded signals into IF signals, and an RF up-converter 608 converts the IF signals into RF signals based on a reference frequency supplied from the local oscillator 611. A high-power amplifier 609 amplifies the RF signals and the amplified RF signals are transmitted through a transmission antenna 610.
The on-channel repeater shown in FIG. 6 improves the multipath and noise removing ability, which is a problem of the repeaters illustrated in FIGS. 3 to 5, by employing the equalizing and FEC decoding unit, but it increases time delay from conventional microsecond-level to millisecond-level because it includes the FEC encoder and the FEC decoding unit. Furthermore, since the feedback signals generated due to ambiguity of a standard trellis encoder of the FEC encoder is not removed, there is a problem in that the transmission output power is restricted.
FIG. 7 is a block diagram illustrating a conventional on-channel repeater employing an equalizer. Referring to FIG. 7, a reception antenna 701 and an RF receiver 702 receive RF signals transmitted from a main transmitter. An IF down-converter 703 converts the received RF signals to IF signals based on a reference frequency supplied by a local oscillator 710, and a demodulator 704 demodulates the IF signals into baseband signals. An equalizer 705 eliminates noise and multipath signals caused on a transmission path between a main transmitter and the on-channel repeater and feedback signals generated due to low isolation of transmission and reception antennas. A modulator 706 converts the baseband signals without noise, multipath signals and feedback signals into IF signals. An RF up-converter 707 converts the IF signals into RF signals based on a reference frequency supplied by the local oscillator 710. A high-power amplifier 708 amplifies the RF signals and the amplified RF signals are transmitted through a transmission antenna 709.
The on-channel repeater of FIG. 7 which employs an equalizer has a problem in that when the electric field strength of the transmitted feedback signals (which are generated due to low isolation of the transmission and reception antennas) is higher than the electric field strength of input signals inputted to the on-channel repeater, the equalizer cannot only remove feedback signals but also it makes an emission, which causes the on-channel repeater to malfunction.
Due to the limitation in the ability of removing feedback signals, which is common among conventional repeaters, the conventional on-channel repeater employing an equalizer has a low applicability and requires a great amount of investment.
Therefore, it is required to develop an on-channel repeater which outputs the same output signals as the output signals of a main transmitter, has a short time delay between the output signals of the main transmitter and the output signals of the on-channel repeater, outputs signals whose characteristics are superior to those of input signals of the on-channel repeater by eliminating noise and multipath signals caused on a transmission path between the main transmitter and the on-channel repeater, and increases transmission output power of the on-channel repeater by eliminating feedback signals generated due to low isolation of transmission and reception antennas (i.e., feedback signals having a higher electric field strength than the input signals of the on-channel repeater) to thereby improve applicability and reduce the amount of investment.